Many industrial processes require that materials, such as liquid materials flowing through a pipe, be maintained at a relatively constant temperature. Elongate strip heaters are often utilized for such purposes. It has been common for such strip heaters to have metallic resistive heating elements, although heaters having flexible electrically conductive polymer elements provide superior characteristics for many applications. Generally, a strip heater may be constructed with a resistive polymer having two elongate conductors enclosed therein such that current flowing between the conductors creates Joules heat within the resistive material. A self regulating heater can be constructed using a positive temperature coefficient ("PTC") material, or a combination of PTC materials and zero temperature coefficient ("ZTC") materials, configured such that heating of the heater strip results in an increased total resistance and thus in a reduction of Joules heat. U.S. Pat. No. 4,654,511, issued to Horsma et al., teaches a method for utilizing several layers of PTC and/or ZTC materials to provide a characteristic temperature curve tailored to the application. By this means, the heater can be designed to optimize the heater for an application. For example, a high initial resistance (useful for limiting inrush current) can be obtained while only marginally sacrificing switching temperature characteristics.
Although there have been many useful variations in strip heater design, there remain applications in which it is desirable to have more precise temperature control and/or greater efficiency than is available through the use of known constructions. Furthermore, although the temperature curve characteristics of a strip heater can be controlled by use of known combinations of PTC and/or ZTC and/or negative temperature coefficient ("NTC") materials, even more precise control of such characteristics would be highly desirable.
To the inventor's knowledge, no prior art strip heater construction has provided highly accurate temperature control while also providing a great amount of heat, as may be required in applications requiring high temperatures and/or large volumes of materials. All prior art devices have either imprecisely monitored material temperature and/or have been incapable of providing sufficient heat for some applications. Also, to the inventor's knowledge, no prior art strip heater construction has provided the ability to precisely control switching temperature range and rate.